The present invention relates to a novel ultraviolet light absorber, an ultraviolet light resistant resin composition containing the above ultraviolet light absorber and a sun block cosmetic material containing the above ultraviolet light absorber.
Organic high molecular weight compounds (to be sometimes referred to as xe2x80x9cresinxe2x80x9d hereinafter) such as plastics, lubbers and fibers are used in large quantity. When these organic high molecular weight compounds are exposed to a light, they are deteriorated in physical properties by ultraviolet light. Therefore, an ultraviolet light absorber is incorporated therein to impart ultraviolet light resistance. Further, a human skin also undergoes sunburn, pigmentation or cytoclasis by ultraviolet light, which induces light-irritable dermatitis or malignancy. Further, the occurrence of geroderma is also advanced. As a preventer for the above problems, a sun block cosmetic material containing an ultraviolet light absorber is commercially available.
The ultraviolet light absorber includes an organic compound-containing ultraviolet light absorber such as a benzotriazole-containing ultraviolet light absorber or a benzophenone-containing ultraviolet light absorber, and an inorganic compound-containing ultraviolet light absorber such as titanium oxide or zinc oxide. The organic compound-containing ultraviolet light absorber is excellent in transparency and also excellent in ultraviolet light absorption. The above type ultraviolet light absorber is therefore used as an ultraviolet light absorber in most cases and the usage thereof is large. In contrast, the inorganic compound-containing ultraviolet light absorber is poor in transparency and it is inferior to the organic compound-containing ultraviolet light absorber in terms of ultraviolet light absorption. The inorganic compound-containing ultraviolet light absorber has been unusually used as an ultraviolet light absorber. However, the inorganic compound-containing ultraviolet light absorber has high safety and it has attracted attention in recent years. Therefore, zinc oxide or titanium oxide having ultrafine particles is developed and transparency or ultraviolet light absorption is improved. As a result, the above zinc oxide or titanium oxide has come to be used mainly in a sun block cosmetic material in place of the organic compound-containing ultraviolet light absorber. Furthermore, recently, a zinc oxide type solid solution has been proposed as a novel ultraviolet light absorber.
In recent years, the standard of living and the average life expectancy are increased so that requirements for the safety of a product have been more and more increased. In other words, the term xe2x80x9csafetyxe2x80x9d refers to non-toxicity or low toxicity and no correspondence to environmental hormones. The safety of the ultraviolet light absorber is also required. It is pointed out that the above-described organic compound-containing ultraviolet light absorber has problems concerning toxicity or environmental hormones. On the other hand, the inorganic compound-containing ultraviolet light absorber is poor in transparency and has a problem of dispersibility in a resin while it is safe. Therefore, it is difficult to use the inorganic compound-containing ultraviolet light absorber in a resin.
A solar spectrum can be divided into the ultraviolet region (290-400 nm), the visible region (400-760 nm) and the near-infrared region ( greater than 760 nm). The ultraviolet region can be further divided into the UVA region, the UVB region and the UVC region. The UVB region is 290-320 nm and the UVB region is a region where the largest effect with regard to the sunburn of a skin is produced. The UVC region is 200-290. The ozone layer absorbs an ultraviolet light in the UVC region and no ultraviolet light in the UVC region reaches the earth""s surface. The UVA region is 320-400 nm. While the UVA region causes sunburn, its sunburn power is smaller than that of the UVB region. For example, the erythema causability of the UVA region is small so that no acute inflammation is caused. Conventionally, there has been therefore used a sun block agent intercepting an ultraviolet light in the UVB region. The use of such a sun block agent delays the occurrence of sunburn recognizable by visual observation, which results in exposure to the sunlight for a long time in many cases. Recently, it is come to be known that an ultraviolet light in the UVA region penetrates a skin and impairs the skin. That is, it is said that an ultraviolet light in the UVA region is the main cause of 30 to 40% of skin cancer. Further, it is also said that an ultraviolet light in the UVA region accelerates skin cancer by inhibiting an enzyme which cures cells impaired by a UVB radiation. Furthermore, it is also reported that an ultraviolet light in the UVA region penetrates a skin deeper than an ultraviolet light in the UVB region, causes a change in a blood vessel, causes an early aging of a skin and adds an additional deleterious effect to the deleterious effect of an ultraviolet light in the UVB region.
In recent years, there are developed fine particles of zinc oxide and titanium oxide and these fine particles of zinc oxide and titanium oxide are used in a sun block cosmetic material. However, the formation of fine particles intensifies cohesion and deteriorates dispersibility and in addition it causes poor extension and deteriorates usability. Further, light resistance is deteriorated by a photocatalyst activity. The formation of fine particles involves these new problems. Further, titanium oxide having ultrafine particles is still poor in transparency and additionally poor in the absorption in the UVA region. Zinc oxide having ultrafine particles has good transparency but it has a problem that the absorption in the UVB region is poor.
JP-A-8-337768 discloses a zinc oxide type solid solution which is represented by the formula (2),
(ZnyM2+z)1xe2x88x92xM3+xO1+x/2 xe2x80x83xe2x80x83(2) 
wherein M2+ is at least one metal selected from the group consisting of Mg, Ca, Ni and Cu, M3+ is Al and/or Fe, and each of x, y and z respectively satisfies 0.2 less than xxe2x89xa60.4, (y+z)=1, and 0xe2x89xa6z less than =0.75,
and which has an aspect ratio of from 2 to 200 and an average secondary particle diameter of 5 xcexcm.
This solid solution has good transparency but it has poor absorption of an ultraviolet light, in particular an ultraviolet light in the UVA region.
It is an object of the present invention to provide an ultraviolet light absorber containing a zinc oxide type solid solution which ultraviolet light absorber absorbs ultraviolet lights in both the UVA region and the UVB region well, and is excellent in hydration resistance and an ultraviolet light resistant resin composition containing the above ultraviolet light absorber.
It is another object of the present invention to provide an ultraviolet light resistant resin composition suitable for providing a resin-containing packaging material for food or the like which packaging material can prevent deterioration of food or the like by ultraviolet light.
According to the present invention, there is provided an ultraviolet light absorber containing, as an active ingredient, a zinc oxide type solid solution which is represented by the formula (1),
(Zn)1xe2x88x92xM3+xxe2x88x92xcex4O xe2x80x83xe2x80x83(1) 
wherein M3+ is a trivalent metal such as Al, Fe, Ce or Ti, x is a number in the range of 0 less than xxe2x89xa60.2, preferably 0.05 less than x less than 0.2, particularly preferably 0.06xe2x89xa6xxe2x89xa60.18, and xcex4 is a cationic lattice defect,
and has an average secondary particle diameter of from 0.1 to 1.5 xcexcm, preferably 0.4 to 1.0 xcexcm, in a plate form and a BET specific surface area of from 5 to less than 20 m2/g or a BET specific surface area of at least 20 m2/g.
According to the present invention, there is further provided an ultraviolet light absorber according to the above, wherein the surface of the solid solution of the formula (1) is coated with at least one oxide selected from oxides of Fe, Ce and Ti.
According to the present invention, there is further provided an ultraviolet light resistant resin composition containing 100 parts by weight of a resin and 0.01 to 10 parts by weight of the zinc oxide type solid solution which is represented by the formula (1) and has an average secondary particle diameter of from 0.1 to 1.5 xcexcm, preferably from 0.4 to 1.0 xcexcm, in a plate form and a BET specific surface area of from 5 to less than 20 m2/g or a BET specific surface area of at least 20 m2/g.
According to the present invention, there is further provided a sun block cosmetic material containing, as an active ingredient, the above zinc oxide type solid solution which is represented by the formula (1) and has an average secondary particle diameter of from 0.1 to 1.5 xcexcm, preferably from 0.4 to 1.0 xcexcm, in a plate form and a BET specific surface area of from 5 to less than 20 m2/g or a BET specific surface area of at least 20 m2/g.